A significant percentage of organ transplants are rejected by the immune system. Symptoms of rejection vary depending upon the organ but a biopsy of the transplant is normally required to ascertain that rejection is indeed happening. Biopsies are painful, introduce the possibility of infection, and require subsequent microscopic inspection of the biopsy tissue for lymphocytes. Acute rejection occurs through several mechanisms, but is dominated by multiplication of T cells which attach to HLA antigens on the donor cells and kill them. It is possible to identify the presence of these T cell lymphocytes in the transplanted organ by non-invasive biomagnetic techniques using introduced magnetic nanoparticles specifically targeted to lymphocytes by antibodies, such as CD8+, which attach to their ligand receptors. Sensitive SQUID magnetic sensors can detect the presence of small amounts of these labeled cells by magnetizing the nanoparticles and subsequent measurement of their remanence fields. This methodology can be used to monitor the status of a transplant for the effects of chemotherapy as well as reduce the need for biopsies.